The present application is a continuation-in-part of U.S. application Ser. No. 897,118 (PKR 2 031), filed Aug. 15, 1986.
The present invention relates to the art of magnetic resonance spectroscopy. It finds particular application in conjunction with imaging body tissue and will be described with particular reference thereto. It is to be appreciated that the present invention is also applicable to slice imaging, volume imaging, motion desensitization, motion imaging, and other types of imaging and analysis of selected regions of animate and inanimate objects.
Heretofore, various imaging sequences have been utilized in medical diagnostic and other magnetic resonance imaging. Commonly, the sequences include the application of radio frequency pulses to induce or drive magnetic resonance of selected dipoles in an image region. Various magnetic field gradients were applied across the image region for spatially or otherwise encoding the magnetic resonance. These gradients commonly included phase encode, read, and slice select gradients applied along mutually orthogonal axes. In spin echo, gradient echo, and other echo techniques, magnetic resonance was excited and the magnetization vectors were focused to converge after a short time in an echo. After one or more echoes occurred, another sequence was commence with different magnetic field gradient encoding. After a sufficient number of encoded views were collected, commonly on the order of 256, the collected views were reconstructed into a magnetic resonance image representation.
The echo techniques generate excellent images. The spin echo technique, for example, generates T2 weighted images which have particularly useful diagnostic value. However, when using echo techniques, a relatively long time interval was required to collect the necessary resonance data.
Steady state magnetic resonance imaging techniques have been developed which improve or reduce the data acquisition time. In nuclear magnetic resonance spectroscopy, steady state resonance was induced by applying periodic, equal, and coherent RF pulses to a nuclear spin system. The free induction decay following each pulse was monitored and analyzed to assess selected spectroscopic data. Analogous imaging techniques have also been developed. Again, a string of periodic, equal, and coherent RF pulses were applied. Between adjacent pulses, the magnetic resonance signal decayed and started to build toward an echo. However, before an echo formed, the next RF pulse was applied. In this manner, a steady state driving of the magnetic resonance spin system was achieved. Magnetic field gradients were applied to encode spatial information and other imaging information in the steady state resonance signals. The magnetic resonance signal was sampled at a selected point between the two RF pulses. The magnetic resonance signal could be sampled closely following an RF pulse before the magnetic resonance signal has decayed too low. Alternately, the magnetic resonance could be sampled after it started building toward an echo immediately preceding the next pulse. However, time had to be accorded between the sampling point and the adjacent RF pulses to accommodate the necessary gradient field pulses. The spatially encoded resonance data was reconstructed into an image representation using Fourier transform or other known reconstruction techniques.
In another imaging technique, a saturation pulse was applied closely preceding each RF pulse to saturate the magnetic resonance signal from flowing material. Images from following gradient echoes had little or no motion artifacts. However, images were prone to artifacts from main field inhomogeneities, variations in magnetic susceptibility, proximity to ferrous objects, and the like.
Another drawback to the prior art techniques resides in the demands on the hardware for applying gradient pulses between the close radio frequency excitation pulses. Moreover, these technique suffer chemical shift, main field inhomogeneity, magnetic susceptibility variation, and ferrous object proximity degradation.
The present application provides a new and improved imaging technique which overcomes the above referenced problems and others.